The cavitation limits of a hydraulic machine are ordinarily established today in the model test with the aid of a model machine which is very much smaller than the large execution, but is nevertheless homogenous (i.e., hydraulically "similar"). By way of the so-called similarity laws it is possible there with very good dependability to infer the behavior of the large execution. By the suction pipe cone of plexiglass and with the aid of a baroscope over the turbine cover, a visual assessment of the cavitation behavior is possible with the model machine.
According to the state of the art the cavitation limits ascertained in the model case converted for the large execution are deposited in the turbine regulator. In modern digital regulators this takes place in the form of characteristic curves, the maximally possible flow being taken as a function of the drop height. Appropriately, instead of the flow there is mostly given the maximal opening of the regulating unit (greatest setting angle of the guide vanes). In many cases there is additionally taken into consideration the influence of the tailwater level on the cavitation limits, i.e. the maximally allowable flow is defined by a characteristic curve family as a function of drop height and tailwater level. The regulator with the aid of these characteristic curves limits the regulating unit and therewith the flow through the machine, so that the admissible limit values cannot in any case be exceeded.
A further refinement is yielded by the consideration of the influence of the tailwater level on the cavitation limits, i.e. the maximally allowable flow is defined by the characteristic curve family as a function of drop height and tailwater level. The regulator with the aid of these characteristic curves limits the regulating unit and therewith the flow through the machine, so that the allowable limit values can in no case be exceeded.
The process hitherto of determining cavitation limit values suffers under a twofold defect. In the first place, there is no exact analogy between the relations of the model test on the one hand and those of the large machine on the other hand. Thus, for example, the water guidance in the model test is not always homogenous, so that in the larger execution there are found in part inflow conditions others than in the model.
In the second place, the cavitation limit values determined in advance by model test and calculation in advance are given firmly once and for all. They do not, therefore, take into account changes that set in during the life of the machine as well as during the life of the entire installation. These changes affect, for example, the geometric contour of the flow-conducting parts (for example of the guide vanes and of the runner blades), or changes that result by reason of sand erosion, rust, repairs, etc. in the course of time, but also changes in the inflow conditions of the machine (for example structural changes, sand banks in the flow bed, changed level of the tailwater level, etc.).
From these there result for the operator of the machine various restrictions and problems. Because of the above-described imponderables there is present, on the one hand, the risk that if the limits are set too generously, unnoticeably progressing cavitation damages will arise.
In order to avoid such damages, it would be possible to choose the cavitation limit values very restrictively. This means, however, that the operating and performance range of the machine is not optimally utilized. This is especially the case when the machine manufacturer, if he must guarantee a certain cavitation behavior for the given limit M, includes safety additions in his reckoning. This has the consequence that at the borders of the operating range there are zones in which possibly a safe operation or at least an operation with calculable risk would be possible, provided that the actual cavitation behavior of the machine is known, i.e. can be determined reliably from state magnitudes detectable by measuring techniques. In this manner, for example, in times of high load on the electric mains it is possible to generate valuable peak energy. Furthermore, in running water power works at high water an increase of the maximal continuous performance is possible.
Further, during operation through the above-mentioned longer-term changes on the machine under some circumstances there is yielded a restriction of the cavitation-free operating range, so that the pre-set limits that were valid for the new machine, are no longer valid for the actual machine state. If the machine continues to be operated in the original manner, this can lead to ever farther progressing damages.
Furthermore, also within the admissible operating range there are unfavorable zones which should be avoided for the sparing of the machine. This was not possible with the apparatus-technical solutions of hitherto, since it was not possible to detect these zones.
An especially serious problem for the operator is presented by the dependence of the erosion danger on hitherto undetectable border conditions. There is to be mentioned, in particular, the sediment concentration. Cavitation in conjunction with high sand concentration leads, in certain operating ranges, to especially severe manifestations of erosion. This results in rapid changes on the blade surface, which contributes to a further reinforcing of the effect. This multiple effect, therefore, is especially dangerous.
For the measurement-technical detection of cavitation, various processes are known. One possibility, for example, is presented by the use of body-sound sensors designed especially for very high frequencies (100 KHz to 1 MHz), which are mounted on the turbine housing. For the further processing, the formation of two characteristic values has proved useful (W. Knapp, C. Schneider, R. Schilling "A monitor system for the acoustic cavitation monitoring of water turbines," 8th International Seminar on "Hydraulic power installations," TU Vienna, 1994). Characteristic value 1 represents the sum effective value of the high-pass filtered time signal. This characteristic value permits a quantitative statement about the cavitation load. Characteristic value 2 is a number signal. The evaluating apparatus here counts the peaks over a pre-defined time window, in which each peak represents the action of a bursting cavitation bubble. This characteristic value is well suited for the detection of the cavitation onset, because at first it rises very steeply and with fully developed cavitation it goes over into a saturation range.